This invention relates to an electronic circuit assembled of various electrical and semiconductor components to provide automatically controlled switching of a two-terminal trigger device. The general use of such a trigger device is for exact triggering of larger, higher-current rated semiconductor switching components such as the triac or thyristor. Several circuits of this nature are used in zero-voltageswitching (ZVS) triggers in AC power solid state relays (SSR), however this circuit is not intended to be limited to only SSR type application, since other uses will become apparent to those having alternative applications.
Another function of the circuit is to cause limiting of the maximum value of current to be conducted between the two main power terminals. Other variations and possible modifications perform similar switching action. All switching action is controlled by an externally supplied current. This circuit possesses a minimum leakage current between the power terminals when the device is not triggered.
In this invention, an electronic circuit comprised of various semiconductor type devices controls the degree and timing of triggering of a power switching device such as to effect controlled electrical conduction through the device. The circuit also prevents accidental switching or "triggering" of the power switching device from random noise or interference voltage on the main power terminals. Several variations of the basic circuit concept are possible in different complexities and functions.
One object of the invention is to achieve a "zerovoltage-switch" (ZVS) function whereby the triggering of the power switching device is restricted to occur only during the interval that the voltage on the main power terminal is less than a predetermined value.
Another object of the invention is to provide a control circuit capable of limiting the maximum value of peak current to be conducted between two main power terminals to provide protection for itself and the power switching device from excessive current.
Still another object of the invention is to provide a trigger control circuit requiring no external source of power other than that available at the two power switching terminals connected to the power triac or thyristor device, and requiring no external components for its function.
A further object of the invention is to improve the thermal stability of the normal type of thyristor or triac switching device by employing a clamped-gate function prior to triggering or, in the case of non-triggering, to employ a clamped-gate function to prevent false triggering after the terminal voltage has exceeded the trigger limit point.
A still further object of the invention is to employ a high-voltage current-limited resistor constructed in the form of a depletion mode MOSFET or JFET as a non-linear resistor to provide the power drive to the switching and control circuits so as to cause a minimum of leakage current during periods of high terminal voltage when non-triggered, yet so as to deliver adequate current and voltage to the switching and control circuits to achieve adequate functions.
Yet another object of the invention is to provide a predetermined minimum requirement of the input control signal level to permit control from relatively low current sensors.
Another object of the invention is to provide means for prevention of false triggering of the switching device from rapid changes of terminal voltage (dV/dt) and also from excessive voltage (overvoltage) applied to the circuit as a transient or sustained condition.
A still further object of the invention is to provide a circuit which will produce a definite voltage transient and sudden current initiation at the instant of switching so as to deliver a positive drive signal to the subsequently triggered device.
Another object of the invention is to provide a circuit having the capability of instant triggering where required in a time of less than one microsecond so as to respond to unusual power factor relationships in the externally triggered circuit.
A still further object of the invention is to provide an adequately functional control circuit requiring a minimum number of high voltage components.
The invention possesses other objects and advantages, some of which, with the foregoing, will be apparent from the following description and the drawings. It is to be understood however that the invention is not limited to the embodiments illustrated and described since it may be embodied in various forms within the scope of the appended claims.